There really aren't a myriad of similar efforts. Eigen is pretty much the only C++ linear algebra library. BLAS is a C library which is the de facto standard, and ATLAS is an alternative implementation of the BLAS API.
Other than this, there really isn't much more to pick from.
Hmm… BLAS started life as Fortran codes (said intentionally to sound like a native Fortean-er).
Saying BLAS is a C library skips over much of the depth and history of its FORTRAN roots. The work that went into achieving numeric stability of this stuff is astounding. Ditto with LAPACK.
The original BLAS library on Netlib was and still is written in Fortran (F77 likely) but includes a C api called CBLAS that calls into the Fortran code. Alternatives like ATLAS or maybe Intels MKL might be written in C/C++ or other languages.
Fair enough. Are there any notable improvements over Eigen? I couldn't see whether Blaze supports dynamic rank tensor operations (Eigen doesn't). Thanks.
Other than this, there really isn't much more to pick from.